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Use of the three cryptographic techniques for secure communication. Or does security provide some very basic protections that we are naive to believe that we don’t need? There are many aspects to security and many applications, ranging from secure commerce 3.0 Common Messaging Protocols and Fortezza payments to private communications and protecting health care information. One essential aspect for secure communications is that of cryptography.

But it is important to note that while cryptography is necessary for secure communications, it is not by itself sufficient. This paper has two major purposes. The first is to define some of the terms and concepts behind basic cryptographic methods, and to offer a way to compare the myriad cryptographic schemes in use today. The second is to provide some real examples of cryptography in use today. DISCLAIMER: Several companies, products, and services are mentioned in this tutorial. Such mention is for example purposes only and, unless explicitly stated otherwise, should not be taken as a recommendation or endorsement by the author. Egyptian scribe used non-standard hieroglyphs in an inscription.

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Ensuring that no one can read the message except the intended receiver. Authentication: The process of proving one’s identity. Integrity: Assuring the receiver that the received message has not been altered in any way from the original. Non-repudiation: A mechanism to prove that the sender really sent this message.

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Key exchange: The method by which crypto keys are shared between sender and receiver. In cryptography, we start with the unencrypted data, referred to as plaintext. The encryption and decryption is based upon the type of cryptography scheme being employed and some form of key. If there is a third and fourth party to the communication, they will be referred to as Carol and Dave, respectively. A malicious party is referred to as Mallory, an eavesdropper as Eve, and a trusted third party as Trent.

Finally, cryptography is most closely associated with the development and creation of the mathematical algorithms used to encrypt and decrypt messages, whereas cryptanalysis is the science of analyzing and breaking encryption schemes. Cryptology is the term referring to the broad study of secret writing, and encompasses both cryptography and cryptanalysis. For purposes of this paper, they will be categorized based on the number of keys that are employed for encryption and decryption, and further defined by their application and use. Primarily used for authentication, non-repudiation, and key exchange. Hash Functions: Uses a mathematical transformation to irreversibly “encrypt” information, providing a digital fingerprint.

As shown in Figure 1A, the sender uses the key to encrypt the plaintext and sends the ciphertext to the receiver. The receiver applies the same key to decrypt the message and recover the plaintext. Because a single key is used for both functions, secret key cryptography is also called symmetric encryption. Secret key cryptography schemes are generally categorized as being either stream ciphers or block ciphers. FIGURE 2: Types of stream ciphers. Self-synchronizing stream ciphers calculate each bit in the keystream as a function of the previous n bits in the keystream.

A block cipher is so-called because the scheme encrypts one block of data at a time using the same key on each block. In general, the same plaintext block will always encrypt to the same ciphertext when using the same key in a block cipher whereas the same plaintext will encrypt to different ciphertext in a stream cipher. Two identical plaintext blocks, then, will always generate the same ciphertext block. CFB mode allows data to be encrypted in units smaller than the block size, which might be useful in some applications such as encrypting interactive terminal input. If we were using one-byte CFB mode, for example, each incoming character is placed into a shift register the same size as the block, encrypted, and the block transmitted.

OFB prevents the same plaintext block from generating the same ciphertext block by using an internal feedback mechanism that generates the keystream independently of both the plaintext and ciphertext bitstreams. ECB, CTR mode operates on the blocks independently. Unlike ECB, however, CTR uses different key inputs to different blocks so that two identical blocks of plaintext will not reuslt in the same ciphertext. Finally, each block of ciphertext has specific location within the encrypted message.

1977 for commercial and unclassified government applications. DES is a Feistel block-cipher employing a 56-bit key that operates on 64-bit blocks. DES was defined in American National Standard X3. Information about vulnerabilities of DES can be obtained from the Electronic Frontier Foundation. DESX: A variant devised by Ron Rivest. By combining 64 additional key bits to the plaintext prior to encryption, effectively increases the keylength to 120 bits. More detail about DES, 3DES, and DESX can be found below in Section 5.

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2 year process to develop a new secure cryptosystem for U. The result, the Advanced Encryption Standard, became the official successor to DES in December 2001. As an aside, the AES selection process managed by NIST was very public. NIST by putting out an open call for new cryptographic primitives. They, too, have approved a number of cipher suites for various applications. 2144, is a DES-like substitution-permutation crypto algorithm, employing a 128-bit key operating on a 64-bit block. 64-bit SKC block cipher using a 128-bit key.

Named for Ron Rivest, a series of SKC algorithms. RC1: Designed on paper but never implemented. RC2: A 64-bit block cipher using variable-sized keys designed to replace DES. It’s code has not been made public although many companies have licensed RC2 for use in their products. RC3: Found to be breakable during development. RC6 was one of the AES Round 2 algorithms.

Key lengths can vary from 32 to 448 bits in length. Blowfish, available freely and intended as a substitute for DES or IDEA, is in use in a large number of products. Twofish: A 128-bit block cipher using 128-, 192-, or 256-bit keys. Designed to be highly secure and highly flexible, well-suited for large microprocessors, 8-bit smart card microprocessors, and dedicated hardware.

Designed by a team led by Bruce Schneier and was one of the Round 2 algorithms in the AES process. MISTY1: Developed at Mitsubishi Electric Corp. 128-bit key and 64-bit blocks, and a variable number of rounds. Designed for hardware and software implementations, and is resistant to differential and linear cryptanalysis. A series of block ciphers designed by James Massey for implementation in software and employing a 64-bit block. SAFER K-64, published in 1993, used a 64-bit key and SAFER K-128, published in 1994, employed a 128-bit key.

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After weaknesses were found, new versions were released called SAFER SK-40, SK-64, and SK-128, using 40-, 64-, and 128-bit keys, respectively. KASUMI is the intended confidentiality and integrity algorithm for both message content and signaling data for emerging mobile communications systems. SEED: A block cipher using 128-bit blocks and 128-bit keys. ARIA: A 128-bit block cipher employing 128-, 192-, and 256-bit keys to encrypt 128-bit blocks in 12, 14, and 16 rounds, depending on the key size. Developed by large group of researchers from academic institutions, research institutes, and federal agencies in South Korea in 2003, and subsequently named a national standard. SMS4: SMS4 is a 128-bit block cipher using 128-bit keys and 32 rounds to process a block.

A family of block ciphers developed by Roger Needham and David Wheeler. TEA was originally developed in 1994, and employed a 128-bit key, 64-bit block, and 64 rounds of operation. Block TEA, was released in 1997. 1 was developed in 1987 for use in Europe and the U. 0 offers no encryption at all. KCipher-2: Described in RFC 7008, KCipher-2 is a stream cipher with a 128-bit key and a 128-bit initialization vector. Using simple arithmetic operations, the algorithms offers fast encryption and decryption by use of efficient implementations.

KCipher-2 has been used for industrial applications, especially for mobile health monitoring and diagnostic services in Japan. Salsa20 uses a 256-bit key although a 128-bit key variant also exists. FPE schemes are used for such purposes as encrypting social security numbers, credit card numbers, limited size protocol traffic, etc. There are several other references that describe interesting algorithms and even SKC codes dating back decades.

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Modern PKC was first described publicly by Stanford University professor Martin Hellman and graduate student Whitfield Diffie in 1976. PKC depends upon the existence of so-called one-way functions, or mathematical functions that are easy to compute whereas their inverse function is relatively difficult to compute. Now suppose, instead, that you have a number that is a product of two primes, 21, and you need to determine those prime factors. You will eventually come up with the solution but whereas calculating the product took milliseconds, factoring will take longer. The mathematical “trick” in PKC is to find a trap door in the one-way function so that the inverse calculation becomes easy given knowledge of some item of information. Generic PKC employs two keys that are mathematically related although knowledge of one key does not allow someone to easily determine the other key. One key is used to encrypt the plaintext and the other key is used to decrypt the ciphertext.

Because a pair of keys are required, this approach is also called asymmetric cryptography. In PKC, one of the keys is designated the public key and may be advertised as widely as the owner wants. The other key is designated the private key and is never revealed to another party. It is straight forward to send messages under this scheme. Suppose Alice wants to send Bob a message. Bob decrypts the ciphertext using his private key. Ronald Rivest, Adi Shamir, and Leonard Adleman.

RSA today is used in hundreds of software products and can be used for key exchange, digital signatures, or encryption of small blocks of data. RSA uses a variable size encryption block and a variable size key. Diffie-Hellman: After the RSA algorithm was published, Diffie and Hellman came up with their own algorithm. D-H is used for secret-key key exchange only, and not for authentication or digital signatures. More detail about Diffie-Hellman can be found below in Section 5. A PKC algorithm based upon elliptic curves. ECC can offer levels of security with small keys comparable to RSA and other PKC methods.

More detail about ECC can be found below in Section 5. A set of interoperable standards and guidelines for public key cryptography, designed by RSA Data Security Inc. Cramer-Shoup: A public key cryptosystem proposed by R. LUC: A public key cryptosystem designed by P. Smith and based on Lucas sequences. Can be used for encryption and signatures, using integer factoring. Handbook of Applied Cryptography, by A.

I tried to be careful in the first paragraph of this section to state that Diffie and Hellman “first described publicly” a PKC scheme. PKC scheme is that it allows two parties to exchange a secret even though the communication with the shared secret might be overheard. Ralph Merkle with first describing a public key distribution system that allows two parties to share a secret, although it was not a two-key system, per se. A Merkle Puzzle works where Alice creates a large number of encrypted keys, sends them all to Bob so that Bob chooses one at random and then lets Alice know which he has selected. An interesting question, maybe, but who really knows?

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Because of the nature of the work, GCHQ kept the original memos classified. In 1997, however, the GCHQ changed their posture when they realized that there was nothing to gain by continued silence. Instead, a fixed-length hash value is computed based upon the plaintext that makes it impossible for either the contents or length of the plaintext to be recovered. Let me reiterate that hashes are one-way encryption. A series of byte-oriented algorithms that produce a 128-bit hash value from an arbitrary-length message.

Designed for systems with limited memory, such as smart cards. Developed by Rivest, similar to MD2 but designed specifically for fast processing in software. MD4 but is slower because more manipulation is made to the original data. FIPS 180-4 The status of NIST hash algorithms can be found on their “Policy on Hash Functions” page. SHS: SHA-1 plus SHA-224, SHA-256, SHA-384, and SHA-512 which can produce hash values that are 224, 256, 384, or 512 bits in length, respectively. SHA-3 is the current SHS algorithm.


Although there had not been any successful attacks on SHA-2, NIST decided that having an alternative to SHA-2 using a different algorithm would be prudent. RIPEMD-160 was designed by Hans Dobbertin, Antoon Bosselaers, and Bart Preneel, and optimized for 32-bit processors to replace the then-current 128-bit hash functions. Seberry, a hash algorithm with many levels of security. HAVAL can create hash values that are 128, 160, 192, 224, or 256 bits in length. Tiger: Designed by Ross Anderson and Eli Biham, Tiger is designed to be secure, run efficiently on 64-bit processors, and easily replace MD4, MD5, SHA and SHA-1 in other applications. D2k hash is a root hash of an MD4 hash list of a given file. Hash functions are sometimes misunderstood and some sources claim that no two files can have the same hash value.

This is in theory, if not in fact, incorrect. Consider a hash function that provides a 128-bit hash value. There are, then, 2128 possible hash values. The difficulty is not necessarily in finding two files with the same hash, but in finding a second file that has the same hash value as a given first file. Since there are more than 7 billion people on earth, we know that there are a lot of people with the same number of hairs on their head. Finding two people with the same number of hairs, then, would be relatively simple.

Alas, researchers in 2004 found that practical collision attacks could be launched on MD5, SHA-1, and other hash algorithms. MD5 Collisions: The Effect on Computer Forensics. Cryptographic hash standards: Where do we go from here? The Impact of MD5 File Hash Collisions on Digital Forensic Imaging. The Impact of SHA-1 File Hash Collisions on Digital Forensic Imaging: A Follow-Up Experiment. Finding MD5 Collisions – a Toy For a Notebook. Law Is Not A Science: Admissibility of Computer Evidence and MD5 Hashes.

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The first collision for full SHA-1. MD5 collisions and the impact on computer forensics. Collisions for Hash Functions MD4, MD5, HAVAL-128 and RIPEMD. Hash libraries, aka hashsets, are sets of hash values corresponding to known files. A hashset containing the hash values of all files known to be a part of a given operating system, for example, could form a set of known good files, and could be ignored in an investigation for malware or other suspicious file, whereas as hash library of known child pornographic images could form a set of known bad files and be the target of such an investigation. Rolling hashes refer to a set of hash values that are computed based upon a fixed-length “sliding window” through the input.

As an example, a hash value might be computed on bytes 1-10 of a file, then on bytes 2-11, 3-12, 4-13, etc. Fuzzy hashes are an area of intense research and represent hash values that represent two inputs that are similar. Fuzzy hashes are used to detect documents, images, or other files that are close to each other with respect to content. Jesse Kornblum for a good treatment of this topic. So, why are there so many different types of cryptographic schemes? Why can’t we do everything we need with just one?